In the blowing of thermoplastic synthetic resin foil or film, a tube is extruded in a thermoplastic state and is blown to form a foil balloon, the expansion of which reduces the wall thickness to that of a foil or film, the balloon being normally closed at an end thereof opposite the end into which the air is blown to expand the balloon.
The apparatus for this purpose can include a blowing head having an annular outlet gap through which the plastic tube emerges.
Outwardly of this tube an outer cooling device is provided which serves to cool the expanding balloon along its exterior. Within the tube and balloon, an inner cooling unit is provided which supplies the inner cooling air and the air which expands the balloon. The air within the balloon can be withdrawn by a central discharge tube, thereby allowing control of the balloon expansion.
In earlier apparatus for this purpose, the internal cooling unit comprised a primary air guide which formed an annular air outlet with an annular portion of the blowing head. A secondary air guide system was also provided and that secondary air guide system could be adjusted axially along the discharge tube.
An apparatus of this type is described in German Patent Document DE 29 26 262 A1 and in that system the primary air guide and the secondary air guide are separate parts which are individually mounted on the discharge tube. They are axially adjustable relative to one another and relative to the blowing head portion in both directions.
In this arrangement, moreover, the adjustability of the primary air guide on the one hand and of the secondary air guide on the other afforded two degrees of freedom which required careful adjustment relative to one another. As a consequence, the control of the primary air flow rate on the one hand of secondary air flow rate on the other for different operating conditions and for different blown foil products and foil blowing operations, was inordinately complex. In practice, such control could not be maintained in a real time manner to insure reliable production of uniform films.